Combustion gas generated in combustion equipment such as an internal combustion engine, a boiler, or a gas turbine is emitted through an exhaust system. As the regulations of exhaust emissions are strengthened, the combustion equipment has been improved, e.g., by modifying the fuel composition, circulating the exhaust gas back into the combustion equipment, and enhancing the combustion process. On the other hand, there has been an attempt to remove harmful components (target components) from the exhaust gas that comes out of the combustion equipment by a dry catalytic treatment apparatus.
The use of nonthermal plasma, which is characterized by extremely high electron temperature and low gas temperature, is being studied as a means for treating nitrogen oxides (NOx) and cleaning the exhaust gas, and a PPCP (pulse corona induced plasma chemical process) or a hybrid method (see the following Patent Document 1) has been proposed. The majority of NOx contained in the exhaust gas is in the form of nitric oxide (NO). In the PPCP, the exhaust gas enters a nonthermal plasma reactor, where NO reacts with an ammonia gas and is converted into ammonium nitrate particles. The ammonium nitrate particles are collected by an electric precipitator. In the hybrid method, NO is oxidized almost completely to nitrogen dioxide (NO2) without ammonia, and then is reduced to nitrogen (N2) and water in a wet chemical processing apparatus, which is provided separately.
In general, the exhaust gas concentrations are likely to be low. Therefore, considerable energy per unit concentration or flow rate is required to treat the exhaust gas efficiently using plasma. This increases the running cost. Thus, technologies that allow the target components first to be adsorbed with an adsorbent, and then desorbed and concentrated with plasma have been proposed (see the following Patent Documents 2 and 3).
Patent Document 1: JP 2000-117049 A
Patent Document 2: JP 11(1995)-114351 A
Patent Document 3: JP 2001-300257 A
However, when the conventional exhaust gas treatment apparatus is applied to stationary combustion equipment (e.g., a boiler or gas turbine) or mobile combustion equipment (e.g., a diesel car), there are several disadvantages: the efficiency is relatively low; the cost is high; a significant amount of water is required; and the apparatus becomes larger in size. Accordingly, a dry chemical processing apparatus should be used as much as possible, instead of the wet chemical processing apparatus as disclosed in Patent Document 1.
A catalytic treatment such as dry selective catalytic reduction (SCR) also has some problems. The catalyst itself includes a noble metal and is relatively expensive. The reserves are limited, and the catalyst life is rather short. Moreover, the catalyst may be affected by poisoning when sulfur is present in the fuel, or responsible for clogging that raises the back pressure and lowers the combustor performance. Therefore, it would be desirable to avoid using the catalyst.
In a so-called diesel engine, light oil is used as a fuel, and the fuel ignites under high compression. The diesel exhaust gas contains a large amount of oxygen, which makes it difficult to use a three-way catalyst that has been utilized widely for gasoline cars.
The SCR or PPCP generally uses ammonia along with a catalyst or plasma in the NOx treatment. However, the ammonia is difficult to handle and hazardous to people's health. There is also a risk of leakage from the apparatus or release of unreacted ammonia gas into the atmosphere. Thus, it is desirable not to use ammonia.
Moreover, the PPCP causes a significant amount of combustible ammonium nitride particles. Therefore, when the PPCP is applied to the mobile combustion equipment (e.g., a diesel car), it will pose a problem of the ammonium nitride particles removal.
The exhaust gas emitted from the combustion equipment generally contains oxygen with a volume concentration of about 2 to 10%. Therefore, the application of plasma to the exhaust gas in a plasma reactor hardly decreases NOx (=NO+NO2), but simply oxidizes NO to NO2. Thus, it does not help to reduce environmental pollution. Further, an additional process such as the hybrid method or PPCP is necessary to remove the resultant NO2.
In the method of Patent Document 2, when plasma including oxygen (O2) is used for desorption of NOx or the like, the desorbed amount is small and the efficiency is low. As a method for treating the desorbed gas, if the desorbed gas is a combustible gas such as volatile organic compounds (VOCs), it can ignite and burn easily. However, if the desorbed gas is a non-combustible gas, the treatment method has not been established yet.
In the method of Patent Document 3, the exhaust gas first is cooled so that a harmful gas is adsorbed with an adsorbent, and then the harmful gas is decomposed directly by plasma that is generated in the exhaust gas. However, this method includes neither desorbing the harmful gas nor regenerating the adsorbent. Moreover, since the method does not use nitrogen plasma, the treatment efficiency is not high, and long-time use of the adsorbent also is a problem.